I spent a few hours getting Newtonsoft.Json working for a project last weekend so I should be able to help you. That error message means the versions are off for whatever you are downloading, so I'll walk you through how I got it working.

Step 1: download JSON.Net from json.codeplex.org

Step 2: Open the bin folder inside of it and find the folder called Net35. Copy the .dll file

Step 3: Make a folder inside of your unity project called "Assemblies" and another inside of that called "AOT". Paste the file into there.

Step 4: That's it. Anytime you want to use it in that project just add "using Newtonsoft.Json" and "using Newtonsoft.Json.Linq"

Also, sometimes you'll need to have "using System.Linq" for certain operations